Single crystal silicon, which is the starting material in most processes for fabricating semiconductor electronic components, is commonly prepared according to the so-called Czochralski (Cz) process. In this process, polycrystalline silicon, or polysilicon, is charged to a crucible and melted, a seed crystal is brought into contact with the molten silicon, and a single crystal (also referred to herein as monocrystalline) ingot is grown by relatively slow extraction. After formation of a neck is complete, decreasing the pulling rate and/or the melt temperature enlarges the diameter of the crystal until a desired or target diameter is reached. The generally cylindrical main body of the crystal, which has an approximately constant diameter, is then grown by controlling the pull rate and the melt temperature while compensating for the decreasing melt level. Near the end of the growth process, but before the crucible is emptied of molten silicon, the crystal diameter is gradually reduced to form an end-cone. Typically, increasing the crystal pull rate and heat supplied to the crucible forms the end-cone. When the diameter becomes small enough, the crystal is then separated from the melt.
To produce semiconductor grade single crystal silicon, and more specifically, large, substantially defect-free crystals (e.g., crystals grown in a twenty-eight inch diameter crucible), the behavior of a solidification interface, which includes a peripheral edge of the crystal being grown, must be controlled. The solidification interface of the crystal being grown is also referred to herein as a melt-solid interface. A shape of the melt-solid interface is an important factor in obtaining a suitable process window for producing single crystal silicon.
Magnetic fields in various configurations have been used in the growth of silicon by the Cz process to modify the melt flow in order to control the incorporation of impurities and point defects. Typically, static or quasi-static fields are employed to create force fields which retard the melt motion established by the combination of thermal buoyant forces and rotations in an axisymmetric crystal growing system. The resultant melt flow is then determined by the design of the thermal environment, the rotations in the crystal puller, and the passive retarding force field. Because the design of the thermal environment is not readily modified, the thermal buoyant forces are not readily modified, and the process flexibility is therefore limited. It would be beneficial to have an additional control mechanism available to modulate the net body forces in the melt in order to establish melt flow patterns that create desired heat fluxes without modifying the hardware-dominated thermal environment.
Accordingly, improved control of the melt flow during the crystal growth process is desired to provide increased process flexibility for production of single crystal silicon.